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We report correlated two-electron ab initio calculations for the hydrogen molecule H 2 in interaction with intense ultrashort laser pulses, via a solution of the full three-dimensional time-dependent Schrödinger equation. Our results for ionization and excitation probabilities (at 800 and 400 nm) as a function of internuclear distance R show strong evidence of enhanced ionization, in both single and double ionization, as well as enhanced excitation, in single and double excitation, as the internuclear distance R increases from the equilibrium value R e . The enhancement of all these molecular processes exhibits a maximum at a critical distance R c , which can be predicted from simple electrostatic and recollision models.

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We study enhanced single and double ionizations and enhanced single and double excitations in the nonsymmetric two-electron diatomic molecular ion HeH(+) in an intense ultrashort laser pulse linearly polarized along the internuclear axis (z axis). We solve a three-dimensional time-dependent Schrödinger equation, TDSE, via correlated two-electron ab initio calculations within the fixed-nuclei approximation. A complex scaling method is used for calculation of both single and double ionizations. These nonperturbative processes increase with large internuclear distance R and reach a maximum at some critical distance R(c) and decrease by further increase of R. This enhanced ionization (EI) at R(c) is accompanied by enhanced single and double excitation processes. Furthermore, EI is stronger when the permanent dipole moment of the molecule and the electric field at the peak of the laser pulse are antiparallel than when they are parallel. We predict analytically the R(c) at which the enhancement of all these molecular processes happens in HeH(+) from a simple quasistatic model and investigate the effect of Carrier Envelope Phase on these nonlinear nonperturbative processes.

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